FIG. 1 is a schematic circuit diagram of a conventional constant-current control circuit. Referring to FIG. 1, the control principle of the constant-current control circuit includes: a current in a switching tube S1 is sampled by a first sampling resistor Rs, and then the sampled current is input to a Cs terminal of a control chip IC. In the chip IC, the sampled current signal input from the Cs terminal is compared with a reference signal, and a driving signal is output via Dr to a control terminal of the switching tube S1 according to the comparison result; therefore, by an adjusting of the control chip IC, a current peak in the switching tube may be equal to a current value configured by the reference signal and the first sampling resistor Rs, hence the current peak in the switching tube may be controlled. Due to the conversion of the main circuit, the peak of the current output from the main circuit may be controlled indirectly; hence a constant current control may be realized.
For the constant current control circuit in which the output current is kept constant by controlling the peak current of the main switching tube, the output of the constant current control circuit is not feedback, hence the circuit has the characteristics of fast dynamic response, simple structure and low cost and is widely applied in low-power constant current circuit.
However, in order to reduce the harm caused by harmonic an d reactive power, in certain application scenarios, a higher power factor (PF) is often required. In this case, the power factor correction circuit needs to be added in the constant current control circuit, where a valley-fill power factor correction (PFC) circuit is adopted most commonly.
FIG. 2 is a schematic structure diagram of a conventional valley-fill PFC circuit. The principle of the valley-fill PFC circuit may be summarized as: the peak portion of the input Sine voltage charges via D3 the capacitors C1 and C2 which are connected in series; in a case that the input Sine voltage is lower than the peak voltage, the capacitors C1 and C2 may be discharged in parallel via D1 and D2. This traditional valley-fill PFC circuit may correct the power factor to some extent, but the PF value is often low due to the distorted input current; hence in certain scenarios where a higher PF value is required, a simple valley-fill PFC circuit may not meet the requirement.
In summary, the conventional constant current control circuit and the conventional valley-fill PFC circuit thereof has the problem of low PF value due to the distorted input current. Therefore, an improved technical solution is desirable to solve this problem.